Method of making polyethylene foam

ABSTRACT

A PROCESS FOR PREPARING POLYETHYLENE FOAM CONSISTING ESSENTIALLY OF FORMING A MIXTURE OF SOLID POLYETHYLENE AND NORMALLY SOLID, HEAT DECOMPOSABLE ORGANIC FOAMING AGENT HAVING A DECOMPOSITION TEMPERATURE AT LEAST ABOUT 10*C. ABOVE THE MELTING POINT OF THE POLYETHYLENE MATERIAL; SUBJECTING SAID ADMIXTURE TO HIGH ENERGY IONIZING RADIATION TO PRODUCE A PERCENT GEL OF ABOUT 30 TO ABOUT 80; HEATING THE IRRADIATED BODY IN STAGES, FIRST AT A TEMPERATURE SLIGHTLY BELOW THE DECOMPOSITION POINT OF THE FOAMING AGENT AND IMMEDIATELY THEREAFTER RAPIDLY HEATING TO A TEMPERATURE WELL ABOVE THE DECOMPOSITION POINT TO FORM AN EXPANDED BODY AND COOLING THE HEATED BODY TO A TEMPERATURE BELOW THE FREEZING POINT OF THE POLYETHYLENE.

United States Patent 01 free U.S. Cl. 2602.5R 5 Claims ABSTRACT OF THEDISCLOSURE A process for preparing polyethylene foam consistingessentially of forming a mixture of solid polyethylene and normallysolid, heat decomposable organic foaming agent having a decompositiontemperature at least about C. above the melting point of thepolyethylene material; subjecting said admixture to high energy ionizingradiation to produce a percent gel of about 30 to about 80; heating theirradiated body in stages, first at a temperature slightly below thedecomposition point of the foaming agent and immediately thereafterrapidly heating to a temperature well above the decomposition point toform an expanded body and cooling the heated body to a temperature belowthe freezing point of the polyethylene.

This application is a continuation of my copending application, Ser. No.307,340, filed Sept. 9, 1963, now Pat. No. 3,432,447.

This invention relates to foamable plastic bodies, foams preparedtherefrom, and to methods of making each. In particular, the inventionrelates to foamable bodies of polyethylene material, polyethylene foamsprepared therefrom and methods of making.

Polyethylene foams are known in the art. Depending upon theirproperties, particularly their density (which may range from as low asabout .03 gram per cubic centimeter to about 0.8 gram per cubiccentimeter), they find wide utility in various industries. Thus, foamedpolyethylene has been used as electrical insulation, thermal insulation,gasketing and sealing material (for cable buoys, life jackets and thelike) and for a wide variety of other purposes.

It is an object of this invention to provide improved foamablepolyethylene bodies having utility in one or more of the above describedareas.

It is another object of this invention to provide methods for preparingthe improved foamable polyethylene bodies.

A specific object of the invention is to provide methods for preparingfoamable polyethylene bodies which upon application of heat and in theabsence of restraint will exhibit three dimensional expansion to formfoamed polyethylene bodies having excellent resistance to thermaldeformation; a uniform smooth surface skin; and a uniform unicellularstructure wherein individual cells are substantially spherical andextremely small.

Still another object of the invention is to provide new and improvedpolyethylene foams characterized especially by uniform, unicellular,substantially spherical and extremely small cells.

Another object is to provide methods for making and for shaping suchfoams.

Still other objects and advantages of the invention will become apparentto those skilled in the art in view of the following more detaileddescription.

It has been discovered that improved foamable poly- Patented July 13,1971 ethylene bodies can be prepared by (a) forming a homogeneousadmixture of (1) a normally solid polyethylene material and (2) fromabout 1 to about 15 parts (by weight) per 100 parts (by weight) of thesaid material of a normally solid, heat decomposable, organic foamingagent having a decomposition temperature at least about 10 centigradeabove the melting point of the polyethylene material while (b)maintaining the materials being admixed below the decompositiontemperature of the foaming agent; (c) subjecting the admixture to highenergy ionizing radiation in an amount corresponding to a total dosageof from about 10 to about 50 megarads and sufiicient to provide apercent gel of from about 30 to about percent in the irradiatedmaterial.

As an optional ingredient there may be added to any of the aboveadmixtures from about 5 to about 40 parts (by weight) per parts (byweight) of the polyethylene material of a normally solid thermoplastichomopolymer or copolymer of monovinyl aromatic hydrocar-bns. Typicalexamples of such optional additives are homopolymers of styrene,ethylvinylbenzene, isopropylstyrene, vinylxylene, vinyltoluene andcopolymers of any two or more of these aromatic hydrocarbons. Theaddition of any of these additives significantly increases the stiffnessof foamed shapes (e.g., trays, bowls, cups, etc.) made from the foamablebodies of this invention and thus broadens the range of usefulapplications for the foamable bodies. In addition, under certainconditions, the additives assist in producing foams having smalleraverage cell size. The preferred additive polymer is polystyrene.

The foamable polyethylene bodies of the invention consist essentially ofthe irradiated homogeneous admixtures described above.

The new and improved polyethylene foams of this invention are preparedby heating the foamable bodies above the decomposition temperature ofthe foaming agent for a sufficient period of time to decomposesubstantially all of the foaming agent and then cooling the foam totemperatures below the solidification temperature of the polyethylenematerial therein.

The term polyethylene material as used herein is intended to includenormally solid homopolymers of ethylene; copolymers of ethylene with oneor more alpha mono-olefins having from 3 to 8 carbon atoms, e.g.,propylene, butene-l, 4-methyl-pentene-l, hexene-l, etc.; and physicalmixtures of ethylene homopolymer or copolymer with another ethylenehomopolymer or copolymer having a different density, or of ethylenehomopolymer or copolymer with one or homopolymers on copolymers of theabove alpha monoolefins. It is required that the polyethylene material,when composed of a copolymer or one of the above-mentioned physicalmixtures, contain a predominant amount, i.e., about 75% or more, byweight, of polymerized ethylene.

The density and molecular weight of the polyethylene material are notcritical factors. Thus, the density may range form about .900 or so upto about .980. Molecular weights may range from about 7,000 or 8,000 upto 1,000,000 or 2,000,000 or higher. For optimum results, however, it ispreferred to use materials having higher densities (e.g., about 0.940 orhigher) or higher molecular weights (e.g., materials having melt indices(as measured in the standard tests) less than about 15.0). In general itcan be said that the higher the molecular weight of the polyethylenematerial, the higher the gel percentage will be at a given radiationdosage. Cell size in the ultimate foam is in turn smaller because of theincreased gel percent. Higher density materials have higher crystallinecontent and hence yield stiffer, tougher foams. It is preferred to usematerials having both higher density and higher molecular weight.

The polyethylene material, per se, is not a part of the presentinvention, since the invention is applicable to various of thepolyethylene materials that are commercially available. Typical examplesof such materials are high, medium or low density polyethylenes producedeither by the high or low pressure processes, both of which processesare now well known to those skilled in the art. Some of the copolymersthat can be used, e.g., ethylene-propylene and ethylene-butenecopolymcrs are also commercially available. The physical mixtures usefulfor this invention include mixtures of ethylene homopolymers havingdiverse densities and/or diverse molecular weights, mixtures of suchethylene homopolymers with various of the ethylene-alpha olefincopolymers or with propylene homopolymer, and mixtures of suchcopolymers with each other or with propylene homopolymer.

The foaming agent used in the practice of this invention is a normallysolid heat-decomposable organic foaming agent which has a decompositiontemperature at least about centigrade above the melting point of thepolyethylene material. It will be apparent that the choice of foamingagent for any particular system will depend upon the polyethylenematerial utilized. Several commercially available materials having highdecomposition temperatures permitting universal utilization in thepractice of this invention are azobisformamide (Celogen AZalso calledazodicarbonamide), N,N-dinitrosopentamethylene tetramine (Unicel NDX),p.p-oxybis (benzenesulfonyl-semicarbazide) (Celogen BH),trihydrazino-symtriazine (THTsee German Pat. 1,001,488),bis-benzenesulfonylhydrazide (BBSH) and barium azodicarboxylate(Expandex 177). Foaming agents with lower decomposition temperatures andhence useful only with low melting temperature polyethylene materialsinclude p,p'-oxybis (benzene-sulfonyl hydrazide) (Celogen),azobis(isobutyronitrile) (Genitron AZDN), and benzene-1,3-disulfonylhydrazide (Porofor B-13).

The use of azobisformamide is highly preferred because of its ease ofincorporation; controllable blowing action; colorless, non-toxic residueand other desirable properties.

In accordance with this invention the amount of foaming agent usedranges from about 1 to about 15 parts, preferably from about 2 to about8 parts, per 100 parts of polyethylene material, all parts beingexpressed as parts by weight.

It will be understood that small amounts, e.g., 0.1% to 2.0% or so (byweight) of other additives such as conventional polyethyleneantioxidants, pigments, and cell nucleation agents can be includedwithin the compositions used to prepare the foamable bodies of thisinvention. Suitable materials serving these purposes are well known tothose skilled in the art.

The foamable polyethylene bodies are prepared by forming an intimate,essentially homogeneous admixture of the above ingredients, optionallypreliminarily shaping the mixture, and then irradiating to the requireddosage and gel content. The homogeneous admixture can be obtained byvarious methods well known to the art. Suitable apparatus includestwo-roll mills, Banbury mixers, twin-screw or single screw extruders ofvarious design and other like hot-mixing machines. In hot mixing, thetemperature at which the homogeneous admixture is formed is below thedecomposition temperature of the solid organic foaming agent.Homogeneous admixtures can also be formed by dry-tumbling theingredients in powdered or particulate form at room temperature orthereabout. The dry-mixed admixture can then, prior to irradiation, beshaped in any desired manner as by compression molding, injectionmolding, extruding, etc., at temperatures below the decompositiontemperature of the foaming agent.

In accordance with the invention the admixture is then subjected to highenergy ionizing radiation. High energy ionizing irradiation is a termwell known in the art. In

accordance with this invention, such irradiation can be accomplishedwith any of a wide variety of sources including radioactive materials,gamma-ray sources, linear electron beam accelerators (e.g., the Van deGraff accelerators), resonance transformer type cathode ray machines,linear electron beam accelerators, atomic piles and waste fissionmaterials, X-ray machines, betatrons, neutron sources and the like.

The time of irradiation is generally not critical. The time is, ofcourse, that sufficient to obtain the necessary dosage and gel content.For extended radiation periods, protection from oxidation will usuallybe required. At higher dosages the time must not be so short as to heatthe body above about 170 centigrade. For the great majority of cases,using conventional radiation sources, the required dosage and gelcontent is obtained long before any temperature rise of this orderoccurs.

The irradiation can be carried out at room temperature or at elevatedtemperatures up to about centigrade. Room temperature is usuallypreferred for economic reasons. Slightly higher efficiencies areobtained at more elevated irradiation temperatures in the noted range,but this is largely offset by the need for protection from oxidation.

The irradiation must in all cases be sufficient to provide a totaldosage of from about 10 to about 50 megarads, preferably from 12 to 30megarads, and at the same time provide a percent gel of from about 30 toabout 80. Bodies irradiated at these dosages provide polyethylene foamshaving very smooth, glossy surfaces; extremely small uniformlydistributed, discrete cells substantially spherical in shape. Lowerdosages, e.g., 8 megarads and below, result in foams lacking one or moreof these highly desirable properties and particularly lacking in finecell size.

The process of this invention provides foamable polyethylene bodieswhich upon foaming have extremely small, discrete uniformly distributedcells. The average cell size is usually less than about .003 inch (.075millimeter) whereas maximum cell size is .008 inch (.20 millimeter) orless. Foams with cell sizes of this order of magnitude have notheretofore been producible with the facility and economy of operationthat is inherent in this invention.

The foamable bodies are foamed by heating the body to a temperatureabove the decomposition temperature of the organic foaming agent. Infoaming the bodies of this invention it is highly preferred to use a twostage heating procedure. This involves preheating the body attemperatures slightly below the decomposition temperature of the blowingagent and then rapidly heating to a temperature above the decompositiontemperature. This rapid transition from below to above the decompositiontemperature gives in general, somewhat lower densities, smaller cellsizes and more uniform cell shape and distribution in the foamsproduced. It should, however, be fully understood that single stageheating does produce satisfactory foam products within the scope of theinvention.

The foamable bodies of this invention, upon heating in the absence ofrestraint, expand three-dimensionally. Nonirradiated bodies of knownpolyethylene compositions containing foaming agent expand primarily inone direction (thickness) when heated under foaming conditions. Increasein length and in width is limited to 10 to 15 percent at the most. Incontrast, irradiated bodies (e.g., sheets) of this invention expand 25percent or more (often up to 5060%) in length and in width, in additionto expanding in thickness. This property is extremely advantageous informing thermoformed (e.g., vacuum-formed) cups, trays, etc., havinguniform thickness distribution.

Cooling of the foam can be accomplished in several different ways. Slowcooling, e.g., by air cooling in ambient atmosphere produces foamshaving in general smaller cell size and greater thermal stability, i.e.,lesser tendency to deform when reheated. Rapid cooling, e.g., byimmediately quenching in a bath of cooling liquid,

will give foams of slightly lower densities, somewhat larger cell sizeand increased tendency to shrink upon reheating. It is apparenttherefore that the cooling method used will depend primarily upon thefoam properties desired.

In quenching, the expanded body is cooled to temperatures below thesolidification temperature (also often referred to as the freezing pointor crystallization temperature) of the polyethylene material as soon asreasonably possible after the body has reached its point of maximumexpansion. As above noted, one satisfactory procedure for quenching isto immerse the foamed body in a bath of cooling liquid, e.g., roomtemperature water.

Formed structures can be prepared from preshaped foamable bodies by,e.g., vacuum forming. In such process it is usually preferred to performthe foaming, forming, and quenching steps substantially simultaneously,that is, in a manner which for practical purposes affords the lowestpossible time lag between these steps. It is most advantageous to foam,immediately form and at the same time quench, i.e., by forming in, or,or around a cold mold.

A preferred method for preparing formed foamed structures involves useof previously foamed shapes, e.g., sheets, using contact heating (e.g.,a heated plate) to heat the foam to forming temperatures and positivepressure (e.g., fluid pressure or a male mold element) to force theheated foam sheet into a cold female mold. This procedure gives formed,foamed shapes having properties essentially the same as those of shapesmade with the above described vacuum forming technique. The formingmethod utilizing previously foamed sheets avoids a possible wrinklingproblem and thus is usually preferred over the described vacuum formingmethod.

The following examples are presented to illustrate the various aspectsof the invention, without limiting the scope thereof other than asdefined in the appended claims. In each of the examples irradiation wasaccomplished using a General Electric one million volt resonanttransformer unit until the indicated dosage was received. The symbol MRis used for the megarad dosage unit.

All parts in the examples are parts by weight unless otherwisespecified.

Gel percentage of the irradiated samples, where shown, were determinedby using the following procedure:

Specimens of irradiated samples weighing between 0.46 and 0.50 gram wereweighed to .1 milligram and to an accuracy of $0.05 milligram. Specimenswere cut into smaller pieces, approximately 1 square centimeter in size,and transferred to a 22 x 80 millimeter single-thickness Whatmanextraction thimble which had been reduced in length by cuttingapproximately millimeters off of the top. As a precautionary measure toinsure against loss of sample in transfer, the thimbles were weighedbefore and after the samples were added. The samples were then extractedover a hour period in an apparatus designed for ASTM D-147 using toluene(analytical reagent grade) as the solvent.

Upon completion of the extraction, the thimbles were removed and avisual inspection made of the gel. If the gel was found to be in acohesive form and capable of total removal with forceps, it wastransferred directly to aluminum weighing cups and dried under reducedpressure in a vacuum oven at 5560 centigrade for a period of no lessthan 48 hours.

If the gel could not be removed without fear of loss to the thimble,then the hot extract was analyzed. It was transferred to evaporatingdishes which had previously been weighed to 0.1 milligram. The flasksfrom which the solution had been transferred were washed twice with20-25 milliliters of hot toluene and the washings added to the solution.Toluene was partially evaporated while cooling under a hood in air priorto drying under the same conditions as was the gel.

Materials balances of 99.610l.l% obtained prior to the investigation andfrom spot checks during the investigation justified the use of solweight for some samples and the gel weight for others. Gel content wasdetermined directly from the gel whenever possible, as it was found tobe a much more convenient procedure.

Determination of cell size in the foamed products was made bymicroscopic examination.

Melt indices of the polyethylene materials were determined in accordancewith the procedure of ASTM- 125852l.

EXAMPLE 1 One-hundred parts of a commercially available, powderedethylenebutene copolymer (about ethylene) having a density of 0.950 gramper cubic centimeter and a melt index of 9.0 were dry-blended with 3parts of commercially available azobisformamide blowing agent(Celogen-AZ) in a Patterson-Kelley twin shell blender for 15 minutes toproduce a homogeneous mixture. A portion of the dry blend wascompression molded at temperatures below the decomposition temperatureof the Celogen AZ into a 9' x 9 x .075-inch plaque in a commercialhydraulic press.

The compression molded plaque was irradiated at room temperature untilit had received a dosage of 16 MR.

A circular portion (about 4 inches in diameter) was die cut from thecenter of the plaque and retained between a pair of circular clamps. Theclamped disc was then preheated at a temperature of about 280 Fahrenheitfor about 2 to 3 minutes after which it was immediately transferred to ahigh temperature (59 Fahrenheit) foaming oven. Foaming occurred withinabout 30 seconds in the high temperature oven. The sample was kept inthe foaming oven for a few seconds to permit substantially completedecomposition of the blowing agent, and was then immediately removedfrom the oven. Quite surprisingly it was noted that during foaming, thedisc expanded in a substantial amount in three-dimensions, i.e., therewas an increase of 40% or more in the diameter of the disc as well as inthickness. Because of the restraint on diametral expansion due to thecircular clamps, the disc expanded into a foamed dome-like bubble. Afterremoval from the oven, the foam was air cooled in the ambientatmosphere.

Samples were cut from the center of the dome for density measurementsand examination of foam structure. The sample had density of 0.47 gramper cubic centimeter calculated from a determination of its volume andweight. The surfaces of the foam were smooth and glossy. It was observedby microscopic examination that the average size of cells in the foamwas less than .001 inch with a maximum cell size of .002 inch. The cellswere substantially spherical in shape and very uniformly distributed.The foam structure was essentially a closed cell structure; i.e.,virtually every call was discrete from other cells.

EXAMPLES 2-45 In the following examples, various polyethylene materialswere homogeneously admixed with azobisformamide in varyingconcentrations by dry blending for 20' minutes (as described inExample 1) and compounding the materials in a two-inch twin-screwextruder. The extruded mixtures were pelletized and re-extruded attemperatures of about 300 Fahrenheit or below to form 15 mil or 30 mil(.015 or .030 inch) elongated sheet, 4 to 6 inches wide.

The sheets were irradiated to varying dosages. Thereafter circular discs(about 4 inches in diameter) were diecut from the irradiated sheet. Eachof these disc samples was foamed and then air cooled in the mannerdescribed in Example 1. In some of the later examples duplicate sampleswere prepared and water quenched after foaming. Densities of foamedmaterials were determined by calculation and pore sizes were determinedby microscopic examination. Percent gel was determined only for someexamples. The gel values noted provides suflicient representativeinformation.

Results are summarized in Table I following:

'IABLEI. FOAMS MADE FROM IRRADIATED HOMOGENEOUS MIXTURES OF POLY-ETIIYLENE MATERIALS AND AZOBISFORMAMIDE Calculated density (grams percubic Radiation centimeter) Average Poly- Amount of dosage cell ethylenefoaming (mega- Percent Air Water size, Cell material agent rads) gelcooled quenched inches 2 uniformity Example:

2 2 8 36 3. Z 12 .30 .003 G d. 4 2 1G 0 .48 001 Excellent. 2 .20 65 001D0. 0. 3 8 .20 .000 None. 7 3 12 20 O02 Fair. 8 3 16 .47 .001 Excellent.EL 3 20 47 001 Do. 10 5 8 23 000 Very poor. 11 A 5 12 23 002 Fair. 1'2 A5 16 .23 001 Excellent. 13.. A 5 20 42 .001 D0. 14 A 7 8 Z25 006 None.15 A 7 l2 18 003 Foot. 10 A 7 .14 001 Excellent. 17 A 7 21 001 Do. 18 B3 .04 74 0005 19 B 3 .06 .86 0005 20 B 7 .72 32 0005 21 B 7 81 54 00052.." B 15 29 20 011 23 B 15 26 16 006 24 13 15 33 16 .0005 25 C 3 41 34010 26 C 3 46 35 00G 27 C 3 57 34 0005 2S D 3 8 33 22 010 29 D 3 12 3?.27 008 30 D 3 20 .47 .006 31" E 3 8 42 3U 016 3 E 3 12 3E) 34 012 33 E 3.20 .49 .41 .0005 34 F 3 8 36 010 35 F 3 12 36 31 00G 36 F 3 20 45 34()01 37.. G 3 8 33 24 017 38.. G 3 l2 33 2E! 008 39" G 3 20 37 l0 003 40H 3 8 .28 28 012 41 II 3 12 32 25 006 42 H 3 20 39 32 001 43" I 3 8 5843 012 44 I 3 12 38 48 005 I 3 20 59 51 005 1 Parts per 100 parts ofpoylethlyene material.

NOTE.POl.V0tllylcnc materials:

A Ethylene-butene copolymer of Example I, .95 density, 0.0 melt index.

B Ethylene-butene copolymer, .05 density, 5.0 melt index. C Ethylenehomopolymer, .90 density, 5.0 melt index.

D Ethylene homopolymer, .033 density, 3.0 melt index.

E Ethylene homopolymer, .06 density, 3.0 melt index.

F Ethylene homopolymer, .96 density, 3.0 melt index, thermally crackedto about 40 melt index. G Ethylene homopolymer, .06 density, 3.0 meltindex. thermally cracked to about 15 melt index. H Ethylene-butanecopolymcr, .95 density, 3.0 melt index, thermally cracked to about 15melt index.

I Ethylene-butene copolymer, .05 density, 0.5 melt index.

EXAMPLES 46-51 TABLE II Parts Per- Average Air Wated of poly- Doscentcell size, cooled quenche r styrene age gel inches density densityExample:

The addition of 10-30% by weight of polystyrene greatly increased thestiffness of the foams produced without materially affecting other foamproperties. Polystyrene in amounts higher than 30% (e.g., 40%) wereunsatisfactory because there appeared to be no significant 7 increase instiffness and an undesired increase in brittleness.

The small cell size of the foams of this invention provides greaterimpact strength, greater flexibility and higher tensile strength ascompared to foams of similar density but with larger pore sizes.

We claim:

1. A process for preparing polyethylene foam comprising:

(a) forming an intimate homogeneous mixture consisting essentially of:

(1) a normally solid polyethylene material; (2) a heat responsivefoaming agent;

(b) forming said mixture into at least one body;

(c) preheating said body uniformly to a particular temperature slightlybelow the decomposition point of the foaming agent and maintaining saidbody at said particular temperature until equilibrium is reached;

(d) in a definite separate part of the heating procedure rapidly heatingsaid already preheated body to a temperature well above saiddecomposition point to form an expanded body, and

(e) cooling the heated body to a temperature below the freezing point ofthe polyethylene material.

2. The process of claim 1 wherein said normally solid polyethylenematerial is subjected to high energy ionizing radiation.

3. The process of claim 2 wherein radiation is carried out in an amountsufiicient to provide a dosage of from about 10 to about 50 megarads andto produce a percent gel of from about 30 to about 80 in the irradiatedmaterial.

4. The process of claim 3 wherein said heat responsive foaming agent isa normally solid heat decomposable organic agent having a decompositiontemperature of at least about 10 C. above the melting point of thenormally solid polyethylene material, said foaming agent present in anamount of from 1 to about 15 parts by weight per 100 parts of saidnormally solid polyethylene material.

5. The process of claim 4 wherein said irradiated, ex-

panded body is then reheated to a temperature suitable 15 for formingand substantially simultaneously formed and quenched.

References Cited UNITED STATES PATENTS 2,948,665 8/1960 Rubens et al204159.2 3,098,831 7/1963 Carr 260-25 3,098,832 7/1963 Pooley et :al.204159.2

